Digital communication systems, such as modem systems, are well known in the prior art. Such systems may employ various line probing or impairment learning techniques that are utilized to determine the characteristics of the communication channel between a transmit device and a receive device. Line probing signals are usually transmitted near the beginning of a start-up or "handshaking" procedure, during which timing synchronization, equalizer training, and other system initialization techniques may be performed.
The specific impairment learning technique and the particular learning signal or sequence used by a given communication system may depend on the design of the system itself. For example, certain line probing sequences may be more efficiently processed by a given system; the precise format of the line probing signal may vary according to the detection scheme utilized at the receiver, the adaptive equalization structure, the impairment learning methodology, or the like. Although line probing protocols for prior art modems may be governed by internationally recognized operating standards, such protocols do not contemplate the use of a flexible line probing or learning sequence that can vary from application to application.
Unfortunately, such rigidly configured impairment learning signals may not be desirable in the context of a pulse code modulation (PCM) modem system, such as a 56 kbps modem system, which may employ any one of a number of different receiver configurations and which may utilize any one of a number of different digital impairment learning techniques. In addition, the use of a single learning sequence may be undesirable where operating standards for a particular technology have not been established; the use of a flexible line probing sequence ensures that a current system can be suitably reconfigured in accordance with future governing protocols.
Line probing techniques in prior art modem systems endeavor to determine digital impairments of the communication channel and select a particular signal point constellation for use during subsequent data transmission. Such techniques may employ a finite number of sub-constellations based on the .mu.-law, A-law, or other conventional signal point constellations employed by the particular telecommunication system. Fixed sub-constellations may be suitable for some applications, however they do not always provide the best signal point constellation for the given modem system and the current operating conditions. In addition, such prior art methodologies may analyze channel impairments in response to a probing signal that does not actually contain individual representations of the signal points utilized by the telecommunication system. Such methodologies may estimate the actual channel response to specific signal points rather than directly measure such responses.
Accordingly, the specific receiver design in a PCM modem system may dictate the particular configuration of the digital impairment learning signal used by the system. However, a learning signal that is effective for one PCM receiver may be unsatisfactory for use with another PCM receiver; such compatibility problems may detract from the performance of these PCM systems. In addition, it may be difficult to detect or utilize prior art line probing signals that are governed by current standards and protocols where such line probing signals are not optimized for use with the specific receiver.